The present invention relates to an optical disk apparatus which executes a formatting to set a sector region in the circumferential direction of a track to a predetermined length and executes reading and writing operations of data by using a peculiar clock frequency every track zone in which the number of sectors in the circumferential direction is equal and, more particularly, to an optical disk apparatus which modulates a read beam in order to suppress a back talk in a read optical system.
In a recording medium of the conventional optical disk apparatus, a constant angular velocity (CAV) format is used. In the CAV format, as shown in FIG. 1, a recording region of an optical disk medium 130 is divided into radial sector regions 354 and physical sector lengths on the inner rim side and outer rim side are different. In such a CAV format, when the optical disk medium 130 is rotated at a constant rotational speed, all of the angular velocities of one sector are constant. Therefore, the reading and writing operations can be executed by a clock of a single frequency. As a result, the physical length on the inner rim side is shorter than that on the outer rim side for the same data amount and a data density in the circumferential direction on the inner rim side is higher than that on the outer rim side.
On the other hand, as a format which increases a memory capacity of the recording medium in the optical disk apparatus, a modified constant angular velocity (MCAV) format has been proposed. In the MCAV format, as shown in FIG. 2, the physical lengths of one sector in the circumferential direction of the track are almost equal. According to the MCAV format, since the sector lengths in the circumferential direction are set to be constant, the number of sectors per track decreases step by step from the outer rim toward the inner rim. Therefore, by collecting the tracks each having the same number of sectors, one zone 356 is formed. Due to this, the number of sectors per track is different depending on each zone. In the case where the optical disk medium of the MCAV format is rotated at a constant rotational speed, in the tracks which belong to the same zone, the angular velocity of one sector is constant. However, between zones, the angular velocity increases from the inner rim toward the outer rim. Therefore, the clock frequency is changed every different zone in the radial direction, thereby setting the densities in the circumferential direction on the inner rim and outer rim to be almost constant. That is, as a clock frequency which is used in the reading and writing operations, the clock frequency such that it becomes higher every .zone from the inner rim to the outer rim is set. The clock frequency is switched in accordance with the zone.
When a read request or a write request from a host computer is received, such an optical disk apparatus executes a seek control for positioning an optical head to a target track on the optical disk medium. At a time of the seek control, the optical disk apparatus which uses the MCAV format sets the clock frequency of the zone corresponding to a track address which is recognized at present and reads the track address. If the track address can be read, the seeking operation is started. If the reading of the track address fails, a retry to switch the clock frequency to the clock frequency of the adjacent zone and to read again is executed. The clock frequency is switched every zone and the track address is read again until the track address can be correctly read.
Flowcharts in FIGS. 3 and 4 show conventional seeking operations. First, in step S1 in FIG. 3, a seeking command from an upper apparatus is received. In step S2, the clock frequency of the zone corresponding to the present track address which is recognized on the basis of a track counter is set into a frequency converter and the track address is read. If the track address can be read in step S3, the processing routine advances to step S12 in FIG. 4 and the number of tracks from the present track address to the target track address is obtained. In step S13, the seeking operation is executed. According to the seeking operation, the number of tracks until the target track is subtracted by "1" every time a track crossing pulse is obtained and the number of remaining tracks is obtained. When the number of the remaining tracks is equal to zero, the arrival at the target track is detected and the control mode is switched from the seek control to a position control(fine control) to allow the light beam to trace the target track. When the seeking operation is completed, the clock frequency of the zone which includes the target track address is selected and the track address is read in step S14. If the track address can be read, the seeking operation is finished as a normality and is shifted to the reading or writing operation.
On the other hand, in step S3 in FIG. 3, in the case where the head position has been deviated into a different zone at the start time of the seeking, even the clock frequency of the zone which is recognized at present is set, the track address cannot be read. In this case, a retry counter NO is counted up by "1" and the reading operation of the track address by the same clock frequency is repeated until the number of reading operations reaches a predetermined number of retry times A in step S3. When the retry operation fails, the clock frequency is switched and changed by changing the zone one by one. Namely, the clock frequency of a zone Z which is obtained by adding "1" to the present zone is set into the frequency converter and the track address is read in step S6. If the track address cannot be read, the retry such that the same clock frequency is switched to another clock frequency by increasing the zone number Z by "1" and the reading operation is again executed is repeated until a retry counter N1 reaches the number of retry times A in steps S7 to S10. When the track address can be correctly read during the retry operations in step S7, the processing routine advances to the seeking operation in FIG. 4. In the case where the track address cannot be read even the clock frequencies of all of the zones are switched, the processing routine is finished as an abnormality. On the other hand, in the case where a defective reading operation of the track address occurs in the read check in step S15 after the seeking operation was completed by the setting of the clock frequency after completion of the seeking operation as shown in steps S12 to S14 in FIG. 4, the same retry process as those in steps S3 to S11 before the seeking operation is repeated in steps S14 to S23.
In the case where the retry process is executed due to the defective reading operation of the track address, when the head stays accidentally in the same zone because of the seeking failure, the track address can be read by the rereading process. When the head jumps to a different zone, however, the reading operation is executed by the retry process while switching the clock frequency and there are problems such that the retry process takes time and the performance deteriorates.
On the other hand, in the optical disk apparatus, in the case where the data recorded on the optical disk medium is reproduced, an error occurs due to a defect on the medium or the like. Therefore, an error correction code ECC is added to the recorded data and by using the error correction code ECC, the error is corrected by an error correction circuit and an error rate of about 10.sup.-12 is accomplished. However, since there are various factors to generate errors, there is a case such that the error cannot be corrected by only the error correction code ECC. As one of the factors of the error occurrence, a back talk as a peculiar phenomenon of the optical disk can be mentioned. This is a phenomenon such that in a light emission control of a laser diode which is used for recording and reproduction, a secondary resonator is constructed between the surface of the optical disk medium and the laser diode and noises are generated. As a method to suppress such back talk noises, a method such that the light from the laser diode is modulated by a high frequency to thereby obtain a modulation light, and when the return light due to the back talk is generated, a light emission amount is reduced, thereby decreasing the secondary resonance amount. Since a modulation depth of the laser diode reduces the life, however, the modulation depth cannot be set to a deep value, so that it is difficult to suppress the back talk completely. Since the generation amount of the back talk changes due to the conditions such as an environmental temperature and the like, there is a problem such that it is difficult to determine the proper modulation amount.